As the computer technology has become sophisticated in recent years, more precise and realistic three-dimensional representations can be realized by CG (Computer Graphics). In the field of CG, actions of a character such as a person or an animal in a virtual space are represented by a skeleton model that is formed with bones connected by joints, and moves the respective bones at the joints.
In such a skeleton model to be operated in a virtual three-dimensional space, the range of motion of the bones connected to the respective joints are set, and the range of motion and the degrees of freedom of actions are varied with the features of the joints. In this manner, more realistic actions are performed. To realize more natural and smoother actions of a skeleton model, a technique of setting the range of motion of joints in a conventional manner by performing coordinate transformation to transform points on a spherical surface into points in a projecting plane, using projection for the respective joints is disclosed (see patent literature 1).